Ukrainian government troops guard a checkpoint just outside Slovyansk, eastern Ukraine, Friday, May 2, 2014. Ukrainian forces launched 'active' operations early this morning in Slovyansk, to oust pro-Russia militants from checkpoints and buildings they have occupied across the city.

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Ukrainian troops have begun an assault on the eastern city of Slovyansk, a move Russia calls a "criminal act" and says could trigger direct Russian military action.

Ukrainian forces launched "active" operations early this morning in the city in the restive Donbass region, to oust pro-Russia militants from checkpoints and buildings they have occupied across the city, reports the BBC. Ukrainian Interior Minister Arsen Avakov described the fighting as "a real battle with professional mercenaries" armed with "heavy weapons."

The government claims that two of its helicopters were shot down, killing a pilot and a serviceman, and that nine occupied checkpoints had been retaken. The BBC was unable to confirm the government's reports. The Wall Street Journal adds that a representative for the rebels in Slovyansk said one of their troops had been killed and another wounded.

The new fighting, while so far resulting in only a handful of deaths, is certain to raise tensions between Russia and Ukraine's interim government. Kiev has been seeking ways to reestablish a modicum of control over the Donbass region of eastern Ukraine, which has largely fallen out of Kiev's control as pro-Russia militants have occupied local government buildings across the area. Russia has consistently warned Kiev against any sort of military action – a warning that carries extra weight thanks to the tens of thousands of troops it has deployed along Ukraine's eastern border.

Many in Kiev worry that the Russian military has already stepped in directly – a concern that reports today will likely fuel. Ukraine's Defense Ministry cited the downing of one of its helicopters by a surface-to-air missile as evidence that the Slovyansk militants are not local citizenry, the Journal notes.

"The fact that shoulder-fired surface-to-air missiles were used against our armed forces clearly indicates that this is a not a 'peaceful protest,' in the Donetsk region," the ministry said in a statement, "but a well-trained and armed professional subversive group committing acts of terrorism and sabotage in order to destabilize the region."

The fighting so far does not appear to have changed the situation in Slovyansk. Vyacheslav Ponomarev, the separatists' self-described "people's mayor," said in a YouTube video published today that "We will not surrender. A lot of people are coming to help us," according to a tweeted translation from freelance journalist Maxim Eristavi.

A separate development heightens the possibility of civilian vs. civilian combat: The Christian Science Monitor reported yesterday that Kiev appears to be giving its approval, tacitly or directly, to the creation of volunteer pro-Kiev militias in eastern Ukraine. Jacob Resneck visited one such militia's camp recently, and saw evidence that the central government was aiding the group of roughly 100 men.

Their commander, a masked 38-year-old who goes by the name Semen Semenchenko, says he was a captain in the Ukraine’s army reserves. But he complains that a lack of political will in Kiev to mobilize local units became frustrating. So he’s formed his own volunteer fighting unit.

“We’re not against the citizens who have different points of view, or are pro-Russia,” Mr. Semenchenko said. “We’re against the politicians and corrupt police and elements of the army who are supporting the separatist movement.” ...

An agent of the Security Service of Ukraine (SBU) – Ukraine’s successor to the KGB – on the scene confirmed that the militias are coordinating their actions, registering their weapons, and operate under an ad hoc agreement with top officials.

“The authorities allow patriotically inclined citizens to form paramilitary groups to defend Ukraine,” he said, declining to give his name.